Secretion of relaxin by the cynomolgus monkey has been studied during the menstrual cycle and during pregnancy. That the corpus luteum was the major source of relaxin during the menstrual cycle and during pregnancy was supported by greater than 80 percent decline in levels found in ovarian venous blood within 30 minutes of luteectomy by dissection. The effect of the conceptus on relaxin secretion was assessed by fetectomy (Fx) of 3 monkeys at 80 days gestation and placentectomy (Plx) at 22 weeks (term = 24 weeks). No change in relaxin secretion was observed following Fx, but Plx resulted immediately in decreased progesterone levels while relaxin levels decreased slowly over several weeks. Simulation of early pregnancy by daily hCG administrations during the nonfertile menstrual cycle resulted in extended luteal phases characterized by decreasing progesterone and increasing and sustained relaxin secretion. A gonadotropin regimen characterized by both its strength and duration appears necessary for relaxin secretion in this primate. in vitro incubation of luteal cells from hCG stimulated ovaries secreted minimal progesterone and significant relaxin.